Because its easier to improve better players? What's the point at starting with a lower floor level? To save a few quid? It's professional football, they're not penny pinchers.
If you had a team of '120 PA' players training at United for 3 years they would absolutely get past that barrier. They wouldn't do it obviously because they'd be relegated and managers sacked in the mean time. But why would you when you can buy the best?
Nobody actually wants that. Not everyone should be Messi and thinking that's how DPA would be is resiging yourself to the idea of a FIFA style system which is silly. Introduce limiters on how far people can grow past their initial PA, more outside effects that lower and raise hidden attributes. Players think they've made it and their ambition goes down, family trauma and their determination goes down. There's some in the game now but quite rare to how it is in real life it feels.
I had a GK on FM21 who had 154 potential and at 21 he reached it. I had only noticed he hadn't been growing about a year later and checked his PA. Now, he was a top level goalkeeper at 21, was a Model Citizen, best facilities and coaches in the world and couldn't grow anymore. Its insanity to think he could get better at anything in his game that young. If there was a player who you rated 154 in the next FM at 21 with a good personality, there's zero chance the devs would then give him a 154 potential to go with it. He had all the assets necessary to progress and he didn't because he's hardcapped.